Process for determining the free-oxygen content of a gas or dust mixture



Dec. 10, 1968' w. J HANSSEN 3,415,626

PROCESS FOR DETERMINING THE FREE-OXYGEN CONTENT OF A GAS OR DUST MIXTURE Filed June 29, 1964 4 Sheets-Sheet 1 le n C H m dg/a Dec. 10, 1968 PROCESS FOR DETERMINING THE FREE-OXYGEN CONTENT OF A GAS OR DUST MIXTURE Filed June 29, 1964 w. J. HANSSEN 3,415,626

4 Sheets-Sheet 2 Mal /an J Hob/V665 W. J. HANSSEN PROCESS FOR DETERMINING THE FREE-OXYGEN Dec. 10, 1968 3,415,626

ONTENT OF A GAS OR DUST MIXTURE Filed June 29, 1964 4 Sheets-Sheet 5 o o 0 Q q; o o o o o o o o o o o O PIC-3.5

[war/V1192 Zia/ 54M [HAM/$5M! W. J. HANSSEN PROCESS FOR DETERMINING THE FREE*OXYGEN Dec. l0, 1968 CONTENT OF A GAS OR DUST MIXTURE 4 Sheets-Sheet 4 Filed June 29, 1964 PIC-3.6

FIG-7 Ruin/Ive lV/ZHELM J HQ/VssEN United States Patent 3 Claims. ((123-432) ABSTRACT OF THE DISCLOSURE A method of determining free-oxygen content of gaseous mixtures by combination of the oxygen with vaporized combustible material. The gaseous mixture is contacted with a vaporizable combustible material until it is saturated, and the combustible material is selected to have a saturation point above the upper explosive point of the mixture. The amount of oxygen is determined by catalyzing the combustion on the surface of a resistor whose resistance is measured. Preferably, the resistor is part of an arm of a resistance bridge circuit and the bridge also includes a resistor having a negative temperature coefficient of resistance. This counteracts the effect of temperature which increases the amount of combustible material in the saturated mixture thereby diluting the oxygen.

The present invention relates to a process for determining the free-oxygen content of a gas or dust mixture. This may, for instance, be done by mixing the mixture with an oxidizable substance and after that introducing it into a reaction chamber in which the oxidizable constituents of the mixture combine with oxygen on a catalytically active measuring wire incorporated in a bridge circuit.

The difficulty attached to this method is how to supply the exact amount of the component required for bringing about the oxidation.

The invention avoids this difiiculty and provides an accurate, portable apparatus. It is characterized in that the mixture is introduced into a reaction chamber together with an oxidizable substance, the amount of oxidizable substance being so chosen that the reaction is effected in the concentration range of the incomplete oxidation and the accompanying heat effect being taken as a measure of the oxygen content.

By preference, the reaction is elfected above the upper explosion limit of the mixture, and the mixture is saturated with the vapor of the oxidizable substance before it is fed into the reaction chamber.

At saturation concentrations much below the upper explosion limit, the output voltage of the circuit used increases very strongly .upon a slight increase of the concentration of the oxidizable substance. This dependency is much less marked in the case of mixtures which, owing to their composition, lie above the upper explosion limit "ice substances may be used for this purpose. Very favorable results are obtained with n-hexane and also with cyclohexane.

By preference, the mixing chamber contains a porous and/or fine-grained inert carrier material. Firebrick is particularly useful since, from the color of the brick, it can be seen whether the amount of liquid contained in it is still sufiicient. If any liquid has to be added, this may, for instance, be done via the suction opening. This addition may also be effected continuously or periodically, e.g., with the aid of a dropping device.

The above procedure makes the dependency on the concentration of the magnitude of the above-mentioned output voltage comparatively small; as a result, a circuit with a negative temperature coefficient can be incorporated in the bridge to compensate for the effect of temperature fluctuations in the mixing chamber on the out put voltage. A complete compensation can be effected for a temperature range of, e.g., 1040 C.

Changes in the pressure of the air being measured can be compensated for by incorporating a variable resistance in the circuit.

The reaction chamber is preferably designed on the diffusion principle, so that the output voltage is little, or not at all, dependent on variations in the rate at which the mixture is drawn in. That is, the reaction chamber is connected to a vessel or conduit containing the sample gas through a narrow passageway so that the sample reaches it only by diffusion.

To prevent condensation in the reaction chamber, this chamber should preferably be heat-insulated and, for instance, be partly made of synthetic material,

The instrument according to the invention can very Well be constructed as a portable apparatus.

It is also possible, in accordance with the invention, to adapt existing gas indicators, e.g., methane indicators, both for the determination of the content of oxidizable components of a gas, particularly of the ambient air, and for the determination of the oxygen content of this air. If the oxygen content is so low that not all of the oxidizable gas can be oxidized, the meter indication will be misleading. However, if the oxygen content is known, such measures may be taken, e.g., addition of oxygen, as will nevertheless enable the content of oxidizable components to be determined accurately. It may, furthermore, be desirable for reasons of safety, for instance, in a coal mine, to know the oxygen content.

In a preferred embodiment of the invention, determination of the oxygen content has been made possible by connecting the inlet of a reaction chamber to a chamber filled with a highly porous carrier material. The carrier material contains a combustible gas and, when the gas whose oxygen content is to be determined is passed through, it gives off an amount of gas which is only slightly larger than would suffice almost completely to bind the oxygen in the form of CO and H 0 during the oxidation. By using only a small excess of oxidizable gas, the amount of CO formed is so small that the catalyst will not be poisoned.

Preferably, the chamber for the carrier material is formed by a replaceable tube which has been hermetically sealed and which is opened only when it is to be used. The design is then extremely simple, there being no reducing or stop valves and no adverse effect of temperature fluctuations. The carrier material may be, for instance, silica gel and the oxidizable gas a saturated aliphatic hydrocarbon which is volatile, preferably at room temperature, for example, pentane. A pentane release of 31- 6% can thus be readily effected during 40 measurements.

Examples of other suitable gases are butane and hexane.

The following equation holds for pentane:

Consequently, the amount of pentane required to bind 21% of O completely /s 21= 2.6%.

It is desirable for measurements of oxygen content in air that the bridge circuit should include a switch for changing over to a second equilibrium position which correlates with a minimum indication of about 13% O and a maximum indication of about 21% These particular limits of the oxygen content have been chosen because determination is of importance only in this range and the accuracy is increased when the whole scale is available for this range, That is, man needs more than 13% of O to remain alive.

The device according to the invention can be constructed as a portable apparatus for carrying out measurements in the underground workings of a mine. It is used for measuring both oxygen contents of 13 to 21% and methane contents of 0 to in the ambient air. The latter percentage is dangerous in that it may cause explosion. However, in places along the roof, in recesses, etc., the methane concentration may be as high as 100%. To be able to measure such concentrations, too, the inlet is connected to a branch pipe having two branches with a known pneumatic-resistance ratio.

If this ratio is 1:19 and air containing no oxidizable components and at least of 0 (6g. exhaled air) is admitted to the branch with the lower resistance and the other branch is connected to the atmosphere to be measured, a 20-fold dilution will take place when the gases are mixed, so that a 100% methane concentration will be indicated by the meter as 5% methane.

This device can also be constructed as a continuous meter and be equipped with an alarm device.

The invention will now be elucidated with reference to the drawing.

FIGURE 1 is a graph which shows the relation between the concentration of n-hexane in air and the output voltage of a bridge circuit without temperature compensation;

FIGURE 2 is a schematic drawing of a circuit according to the invention;

FIGURE 3 is a longitudinal section through a reaction and a compensation chamber;

FIGURE 4 is a section along line IVIV of FIGURE FIGURE 5 shows a buffer space;

FIGURE 6 and 7 show a mixing chamber;

In FIGURE 1 the lower explosion limit is indicated by OE, the upper explosion limit by BE. These limits are at n-hexane concentrations of 1.3 and 7.4%, respectively. It appears that above the lower explosion limit the intensity of the measuring signal initially increases very strongly. After the reversing point of the curve, the fluctuations in the intensity of the measuring signal at first strongly decrease, but the dependency of the signal on changes of the concentration decreases as the concentration increases. The points A, B, C and D indicate the hydrocarbon concentration of the saturated vapor at 10, 11.7, 16 and 20 C., respectively.

By means of a temperature compensation according to the invention, the output voltage can be made completely independent of the concentration according to the line TO. This voltage now varies only with the oxygen content.

FIGURE 2 shows a bridge circuit comprising a source of current 1. Of the four branches one is formed by a fixed resistance 3, one by a heating filament 2 placed in a compensation chamber 26, one by a catalytically active measuring wire 4 fitted in a reaction chamber 5, and one by a network with a negative temperature coefficient, in which a fixed resistance 6 is connected in parallel to a series connection of a variable resistance 7 and a NTC resistance 8 (NTc negative temperature coefficient).

Changes in the air pressure can be compensated for by means of the variable resistance 7. The resistance can be adjusted by hand, but also automatically with the aid of a Bourdon gauge which aiTec-ts a strain gauge. Incorporated in the measuring circuit are a millivoltmeter 9, a changeover switch 10, and an adjustable resistance 11. The source of current 1 can be tested with the aid of an adjustable resistance 12 and the changeover switch 10. A mixing chamber 13 contains brickgrit in which, for instance, cyclohexane has been absorbed. The air can be aspirated by means of a pump 14, via the mixing chamber 13 and the reaction chamber 5, at such a rate that the air in the mixing chamber is completely saturated with the vapor of the liquid.

With reference to a known NTC resistance, the values of the resistances 6 and 7 can be calculated for a relatively large temperature range, e.g., 1040 C., in the mixing chamber, so that within this range the reading of the meter 9 will be constant at a constant oxygen content of the aspirated air.

The reaction chamber 5 consists of a narrow cylinder 15, a plug 16 and a wall 17, which is provided with a slot 18. The reaction chamber is fitted in a metal housing 19, through which the mixture of air and oxidizable substance is passed. To this end, the housing is provided with connecting pieces 20 and 21. Two filters 22 and 23 made of porous, e.g., sintered, metal serve as flame traps, as a result of which explosions, if any, will be confined to the housing 19. Fitted between the cylinder 15 and the housing 19 is a plastic wall 24, which is covered with a heat-insulating material 25, for instance, quartz wool. The wall 24 forms one whole with the abovementioned wall 17 which is provided with the slot 18.

Next to the reaction chamber there is a compensation chamber 26, in which the heating filament 2 has been fitted (see FIG. 2).

The low heat-absorbing capacity of the cylinder 15 and its insulation is an aid in obtaining a proper compensation for the temperature of the gas and a correct influence of the heat of oxidation, and prevents condensation of water. The high thermal conductivity of the housing 19 ensures a similar influence of external temperature variations on both chambers.

The mixing chamber 13 (FIG. 6) is incorporated in a transparent block 27, as are the chambers 28, 29, 30 and 31. The chambers 13, 2830 are series-connected by means of a few openings through which a cotton wick 32 is passed with a tight fit. FIG. 6 is a schematic top view of the block 27, while FIG. 7 shows a developed section through the chambers. The chambers 13, 28-30 are filled with wadding. A gauze basket 33 is placed in the chamber 30. The block is closed with a cover 34, in which a plug 35 is fitted over the basket 33. Between the wall and the wadding, chamber 13 contains brick particles 36. The basket 33 is filled with cyclohexane. The colour of the brick particles 36 contained in the chamber indicates whether the wadding is saturated with cyclohexane. Chamber 31 which is filled with the same dry brick particles, serves as a reference. The NTC resistance shown in FIG. 2 is indicated by 8. This resistance takes the temperature of chamber 13. The arrows indicate the direction of flow of the aspirated air.

The air passes through the chambers 26 and 5 and the pump (FIG. 2). This pump has a delivery of 2 litres per hour and is driven by a very small loud-speaker system. This system is fed by an oscillator circuit connected to a smaller accum'ulator.

After having passed the pump, the exhaust gases are supplied through conduit 37 to a buffer space 38 (FIG. 5) fitted to the housing 39 of the instrument. This buffer space is provided on all sides with diffusion openings 40, which so dilute the gas that it can no longer be combustible.

The invention provides a simple, reliable, cheap, light, and small-sized apparatus.

With the aid of this apparatus, it is possible for instance, to measure the oxygen content of air between 15 and 21% with a deviation smaller than 311%.

An adjustable alarm device which becomes operative as soon as the oxygen content falls below, e.g., 18%, can be added readily.

The meter may also be combined with, e.g., a methane indicator, so that the instrument can, by choice, be made to give an alarm at dangerous methane contents or at dangerous oxygen contents.

What is claimed is:

1. Process for determining the free-oxygen content of a gas and dust mixture comprising saturating said mixture with vapors of a volatile oxidizable substance which has a saturation concentration which, under normal atmospheric conditions, is above the upper explosion limit of its mixture with said gas and dust mixture, reacting the oxygen with said oxidizable substance, the proportions of free-oxygen in said gas and dust mixture and of said oxidizable substance being such that the reaction is effected above the upper explosion limit of the mixture, and measuring the heat liberated by said reaction as an index of the free-oxygen content.

References Cited UNITED STATES PATENTS 2,404,993 7/ 1946 Sullivan 23-255 2,420,430 5/1947 Johnson 23-255 2,470,714 5/1949 Nevius 73-398 XR 2,916,358 12/1959 Valentine et a1. 23254 2,955,922 10/ 1960 Christy 23-254 3,297,943 l/1967 Morgan et al 324-31 OTHER REFERENCES Fisher: Scientific Co. & Eimer & Amend, Modern Laboratory Appliances, 1942.

MORRIS O. WOLK, Primary Examiner.

R. M. REESE, Assistant Examiner.

US. Cl. X.R. 23254 

